Rotary blowers of the Roots type typically include a pair of meshed, lobed rotors having either straight lobes or lobes with a helical twist with each of the rotors being mounted on a shaft, and each shaft having mounted thereon a timing gear. Rotary blowers, particularly Roots blowers are employed as superchargers for internal combustion engines and normally operate at relatively high speeds, typically in the range of 10,000 to 20,000 revolutions per minute (rpm) for transferring large volumes of a compressible fluid like air, but without compressing the air internally within the blower.
It is desirable that the rotors mesh with each other, to transfer large volumes of air from an inlet port to a higher pressure at the outlet port. Operating clearances to compensate for thermal expansion and/or bending due to loads are intentionally designed for the movement of the parts so that the rotors actually do not touch each other or the housing. Also, it has been the practice to epoxy coat the rotors such that any inadvertent contact does not result in the galling of the rotors or the housing in which they are contained. The designed operating clearances, even though necessary, limit the efficiency of the rotary blower by allowing leakage. This creation of a leakage path reduces the volumetric efficiency of the rotary blower.
One known approach to improving pumping efficiency of a rotary blower is the use of a coating with an abradable material. While known supercharger rotor abradable coatings provide, among other things, increased volumetric efficiency of the rotary blower and sufficient lubricating properties, they have been found to exhibit relatively poor corrosion resistance, limiting their use to supercharger applications in which the supercharger is not be exposed to a corrosive environment. For example, known supercharger abradable coatings are generally incompatible with marine engines that operate in a salt water environment, as the relatively high salt content ambient air may corrode the rotors.